Step 1

Position rack in centre of oven, then preheat to 350F. Spray a deep 9-in. baking dish (6-cup capacity) with oil. Boil water in a kettle while preparing cake.

Step 2

Cake: Whisk flour with sugar, cocoa, baking powder and salt in a large bowl. Whisk in milk, butter and vanilla until smooth. Pour batter into prepared dish. Smooth top.

Step 3

Sauce: Whisk granulated and brown sugars with cocoa in a small bowl and sprinkle evenly over batter. Pour 3⁄4 cup boiling water over top without stirring.

Step 4

Bake, covering with foil halfway through, until centre is jiggly when dish is shaken, 25 to 30 min. Transfer to rack to cool, about 5 min. Serve in dishes, spooning sauce over cake. Top with vanilla ice cream, then sift cocoa overtop.